This invention relates to semiconductor lasers and, more particularly, to such lasers pumped by means of an electron beam. Electron beam (e-beam) pumped semiconductor lasers are generally of two types: transverse lasers in which the directions of the e-beam and light beam are orthogonal to one another, and longitudinal lasers in which the directions of the e-beam and light beam are essentially parallel to one another.
(a) Transverse Lasers - This method of pumping semiconductor crystals with a beam of fast electrons to obtain "optical maser" action in the visible and infrared regions of the spectrum was proposed two decades ago by N. G. Basov, Advances in Quantum Electronics, Columbia University Press, page 506, (1961). Experimental work followed soon after, and the first laser action in an e-beam pumped semiconductor was demonstrated by Basov et al, in 1964, Soviet Physics JETP, Vol. 20, No. 4, page 1067, (1965). These researchers used 200 keV electrons to bombard a CdS single crystal that was kept at liquid helium temperature. In the same year, C. E. Hurwitz et al, Applied Physics Letters, Vol. 5, No. 7, page 139, (1964 ) and D. A. Cusano, Solid State Communications, Vol. 2, No. 11, page 353, (1964) obtained lasing in e-beam pumped GaAs. Other workers achieved similar results in InSb, InAs, and GaSb.
In all of these laser demonstrations the crystals were cooled to liquid nitrogen temperature or lower. However, only a year later, in 1965, L. N. Kurbatov et al, Soviet Physics-Doklady, Vol. 10, page 1059, (1966), were able to achieve pulsed laser action in GaAs at room temperature.
In these early lasers, the cavity axis (and hence the light beam) was perpendicular to the pumping electron beam, and as a consequence they are referred to as "transverse lasers". Stimulated emission in transverse lasers is relatively easy to achieve compared to lasing in "longitudinal lasers" where the cavity axis (and hence the light beam) are essentially parallel to the electron beam. The reason for this difference is the shallow penetration of the e-beam into the semiconductor which limits the length of the active region in longitudinal lasers to several micrometers and leaves the remaining, unpumped material is a lossy state. Also, the short distance over which the gain is achieved in longitudinal lasers means that the net gain is relatively small.
Over the past fifteen years, a large variety of semiconductor crystals have been made to lase in the transverse configuration. The lasing wavelengths cover the range of 0.325 .mu.m to 32 .mu.m. Peak power outputs of over 100 watts and power conversion efficiencies of over 20% are not unusual.
Along with these demonstrations and related studies of luminescence properties, several physical effects and semiconductor parameters were investigated. For example, loss mechanisms, carrier migration, material degradation, depth of electron penetration, doping levels and threshold relations, effect of surface treatments on threshold, carrier lifetime versus carrier density, lasing wavelength versus cavity length and as a function of time, far field distributions, effects of scanning velocity, and means for directing the output beam parallel to the e-beam such as distributed feedback structures and V-shaped grooves. Also, the effects of GaAs/AlGaAs heterostructure configurations on lasing threshold were investigated by O. V. Bogdankevich et al, Soviet Journal of Quantum Electronics, Vol. 10, No. 6, page 693, (1980); and Vol. 11, No. 1, page 119, (1981).
In addition, several types of sealed CRT tubes were fabricated and tested, e.g., Yu A. Akimov et al, Soviet Journal of Quantum Electronics, Vol. 10, No. 3, page 368, (1980). These include a demonstration of TV picture projection by (fast) e-beam scanning in one dimension and (slow) rotating-polygon light deflection in the second dimension by V. I. Kozlovskii et al, Soviet Journal of Quantum Electronics, Vol. 5, No. 7, page 865, (1975). Continuous wavelength tuning by scanning the e-beam across a composition-varying semiconductor crystal was also suggested and tested.
Longitudinal e-beam pumped lasers, although much more difficult to make than transverse lasers, are more attractive, especially from a practical point of view, as discussed below.
(b) Longitudinal Lasers - The first stimulated emission in the longitudinal configuration was observed by N. G. Basov, in 1966, Soviet Physics-Doklady, Vol. 11, No. 6, page 522, (1966). Basov et al, modified an accelerator to hit a 100 .mu.m thick, polished slice of GaAs with a focused beam of 150 keV electrons. They observed stimulated emission at liquid nitrogen temperature and even at room temperature. At the end of the same year, W. C. Tait et al, Journal of Applied Physics, Vol. 38, page 3035, (1967) submitted a paper describing stimulated emission from 50 .mu.m thick CdS.sub.0.3 Se.sub.0.7 at liquid nitrogen temperature, using a 50 keV electron beam. Less than two years later, J. R. Packard et al, IEEE Journal of Quantum Electronics, Vol. QE-5, No. 1, page 44, (1969) observed room temperature stimulated emission from 7 .mu.m thick CdS platelets at room temperature with 45 keV electrons. In 1970, F. H. Nicoll, Applied Physics Letters, Vol. 16, No. 12, page 501, (1970) obtained room temperature lasing in 5 .mu.m thick CdS platelets pumped with electrons of only 25 keV energy. In some special cases, where a back aluminum layer was heat-treated to form a bubble, the threshold energy of the pumping electrons (for room temperature lasing) was as low as 9 keV.
In all of these demonstrations and most of the later experiments, the pumping electrons were made to have pulse durations of tens or hundreds of nanoseconds and a very low duty cycle. However, in 1980, V. I. Kozlovskii et al, Soviet Technical Physics Letters, Vol. 6, No. 4, page 198, (1980), were able to obtain CW lasing in a Te-doped GaAs plate which was about 40 .mu.m thick. The semiconductor target was held at liquid nitrogen temperature and was pumped by 50-100 keV electrons. At one point, using 75 keV electrons with a beam current of 6 .mu.A and a beam spot of a few .mu.m in diameter, they measured 12 mW of CW output power. Throughout the years, several sealed tubes were fabricated and tested, some of which operated hundreds or even thousands of hours. See, for example, V. I. Grigor'ev et al, Soviet Journal of Quantum Electronics, Vol. 10, No. 3, page 279, (1980).
The efforts of some of these researchers have resulted in the grant of several U.S. Patents: N. G. Basov et al, No. 3,558,956 (1971); D. A. Campbell et al, No. 3,505,613 (1966); D. A. Campbell et al, No. 3,715,162 (1973); W. C. Tait et al, No. 3,747,018 (1973); J. R. Packard et al, No. 3,757,250 (1973); W. H. Strehlow et al, No. 3,836,224 (1974); and J. R. Packard et al, No. 3,864,645 (1975).
In all of the above-described e-beam pumped longitudinal lasers, the target was a single crystal, in most cases cut from the bulk and then polished on both sides to form a thin smooth wafer. This configuration, however, suffers from several disadvantages: it contains an optically lossy unpumped region, it lacks confinement of excess carriers, and it exhibits nonradiative surface recombination.
The prior art workers were aware of the problem of absorption losses in the unexcited portion of the active region. Tait et al and Packard et al attributed the success of their Group II-VI lasers to phonon-assisted transitions for which the absorption losses in the unexcited region are small. On the other hand, Basov et al in their patent No. 3,558,956 argue as follows: "Owing to the heating of the active region of the film at the depth of penetration of the electron beam, Coulomb interaction between nonequilibrium carriers and lattice polarization, or the interaction of nonequilibrium carriers with phonons, the wavelength of the induced emission is greater than the limit of intrinsic absorption. Therefore, the unexcited regions of the film are transparent to the generated emission and do not produce appreciable losses in the resonator, even though the depth of penetration of the electrons in the semiconductor may be less than the thickness of the film".
However, the gap between the actual threshold current densities reported in the literature and the expected current densities when absorption in the unexcited region is small, suggests that the combined contribution of all of the above-quoted effects is not sufficient for efficient operation. In other words, the threshold in most cases was unnecessarily high due (at least in part) to significant absorption losses in the unexcited part of the cavity. For example, a GaAs target excited by 40 keV electrons should reach threshold at room temperature at a current density of about 20 A/cm.sup.2 when the following conditions are assumed: a carrier lifetime of 1 ns, 5 eV for the generation of one pair (.about.29% internal energy conversion efficiency), 5 .mu.m penetration depth, and 15% overall losses. Under these conditions, the excess carrier density is 2.times.10.sup.18 cm.sup.-3, and the (calculated) gain coefficient is 320 cm.sup.-1. In comparison, Kozlovskii et al, supra, reported an e-beam CW GaAs laser with a threshold current density of about 25 A/cm.sup.2 even though their sample was cooled to 80.degree. K., the electron energy was 75 keV and the transmittance of the output mirror was 1%. Similarly, Packard et al, supra, pumped their room temperature CdS target with 1 mA of 45-50 keV electrons, focused to a spot of about 25 .mu.m in diameter which corresponds to a current density of 220 A/cm.sup.2.
In this context it should be noted that in semiconductors the wavelength at which maximum gain occurs is a function of excess carrier density and is shifted to the short-wavelength side as the carrier density increases. But, because of absorption in the unexcited part of the cavity, the laser is forced to operate at a wavelength region other than that where the maximum gain occurs. This fact is manifested, of course, in increased threshold and reduced efficiency.
With regard to the lack of carrier confinement in prior art longitudinal lasers, we note that the gain coefficient is a very steep function of (minority) carrier density. For example, in GaAs at room temperature the gain coefficient for 1.times.10.sup.18 carriers/cm.sup.3 is 10 cm.sup.-1 whereas for 2.times.10.sup.18 carriers/cm.sup.3 the gain coefficient is 320 cm.sup.-1. Thus, although the diffusion of carriers into the unpumped region increases the length of the active region in the cavity, it may also cause a significant reduction in the overall gain.
The third disadvantage of prior art longitudinal lasers relates to nonradiative recombination at the surface of the semiconductor, particularly at low acceleration voltages. See, for example, A. S. Nasibov et al, Soviet Journal of Quantum Electronics, Vol. 4, No. 3, page 296, (1974). This type of loss, which typically generates heat rather than light, causes a marked increase in threshold and may even prevent laser action completely.